(1) Field of the Invention
The present invention relates to a light receiving device which is used in, for example, a Charge-Coupled Device (CCD) image sensor, and to a method for driving such a light receiving device.
(2) Description of the Related Art
An example of a conventional light receiving device and a method for driving the same, to which the present invention relates, are shown respectively in FIGS. 1A and 1B. The light receiving device shown in FIG. 1A is constituted by a transfer gate element 1 and a reverse biased photodiode 3 which also has an electron charge storage function represented by a capacitor 2. Research is underway on an interline type CCD image sensor which incorporates such a light receiving device and a charge coupled device (CCD).
Actual operation of the light receiving device is explained with reference to FIGS. 1A and 1B. In the diagrams, VL1 is maintained at a 0-volt potential and VH1 at a positive potential. When the light receiving device is combined with a CCD, VH1 is connected to a channel section of the CCD. To a terminal TG which is connected to a gate electrode of the transfer gate element 1 is applied a pulse potential as shown in FIG. 1B. When the VHO that is the ON-voltage is applied to the TG terminal, the photodiode 3 changes to a reverse biased state. Subsequently, when the VLO voltage that is the OFF-voltage of the transfer gate element is applied to the TG terminal, the potential of a PDOUT terminal which is one end (cathode) of the photodiode 3 undergoes changes dependent on the quantity of incident light, as shown in FIG. 1B, with lapse of time. In FIG. 1B, a curved line marked "a" indicates that the quantity of incident light is larger than that of each of "b" and "c", and a curved line marked "c" corresponds to the case where the quantity of incident light is the smallest. With lapse of time T, immediately before the VHO voltage that is the next reset voltage is applied, the potentials become a0, b0, c0, respectively, and the quantities of the incident light can be measured from the differences in these potentials. Also, the quantity of the incident light can be measured by measuring the integrated quantity of the current that flows in from the VH1 when the VHO voltage is applied to the TG terminal. In the case of the interline type CCD image sensor, the quantity of the incident light can be measured by measuring the quantity of charge mixed into the CCD when the VH0 voltage is applied to the TG terminal.
In the conventional light receiving device, there is a problem in that, where the quantity of the incident light is large, the lowering of the potential due to the discharge from the reverse biased state of the photodiode ends prior to the lapse of time T which is a reset period, so that only the signal of the same level is outputted with respect to quantities of the light in the vicinity of the quantity of the incident light. This is a saturation state wherein the incident light is not sensed when the quantity of the incident light is beyond a certain quantity. It is necessary for the light receiving device to satisfy the conditions which do not undergo saturation under an expected measuring range and, for this reason, it becomes necessary also to control the sensitivity (that is, spectral-response characteristic).
Further, in the image sensor incorporating the CCD, other additional conditions are combined for controlling the transfer capability of the CCD, the characteristics involved are unavoidably restricted.
As one method for resolving the problem of saturation, there is a practice wherein a reset period T is made short (the operation called a "shutter mode" in image sensors is one of these methods). However, this method is destructive reading in which the signals thus far accumulated are destroyed by the reset operation. Thus, where the signal from low level illumination is involved and a change in potentials is small, there is a deterioration in the S/N sensitivity.